A geological formation may be penetrated by a borehole for the purpose of assessing the nature of, or extracting, a commodity of commercial value that is contained in some way in the formation. Examples of such commodities include but are not limited to oils, flammable gases, tar/tar sands, various minerals, coal or other solid fuels, and water.
When considering the assessment and/or extraction of such materials the logging of geological formations is, as is well known, economically an extremely important activity.
Virtually all commodities used by mankind are either farmed on the one hand or are mined or otherwise extracted from the ground on the other, with the extraction of materials from the ground providing by far the greater proportion of the goods used by humans.
It is extremely important for an entity wishing to extract materials from beneath the ground to have as good an understanding as possible of the conditions prevailing in a region from which extraction is to take place.
This is desirable partly so that an assessment can be made of the quantity and quality, and hence the value, of the materials in question; and also because it is important to know whether the extraction of such materials is likely to be problematic.
The acquisition of such data typically makes use of techniques of well or borehole logging. Logging techniques are employed throughout various mining industries, and also in particular in the oil and gas industries. The invention is of benefit in well and borehole logging activities potentially in all kinds of mining and especially in the logging of reserves of oil and gas.
In the logging of oil and gas fields (or indeed geological formations containing other fluids) specific problems can arise. Broadly stated this is because it is necessary to consider a geological formation that typically is porous and that may contain a hydrocarbon-containing fluid such as oil or natural gas or (commonly) a mixture of fluids only one component of which is of commercial value.
This leads to various complications associated with determining physical and chemical attributes of the oil or gas field in question. In consequence a wide variety of well logging methods has been developed over the years. The logging techniques exploit physical and chemical properties of a formation usually through the use of a logging tool or sonde that is lowered into a borehole formed in the formation by drilling.
Broadly, in most cases the tool sends energy into the formation and detects the energy returned to it that has been altered in some way by the formation. The nature of any such alteration can be processed into electrical signals that are then used to generate logs (i.e. graphical or tabular representations, or datasets, containing much data about the formation in question).
An example of a logging tool type is the so-called multi-pad micro-resistivity borehole imaging tool, such as the tool 10 illustrated in transversely sectioned view in FIG. 1. In this logging tool a circular array of (in the example illustrated) eight pads 11 each in turn supporting typically two lines of surface-mounted resistivity electrodes referred to as “buttons” 12 is supported on a series of caliper arms 13 emanating from a central cylinder 14. During use of the tool 10 the arms 13 press the buttons 12 into contact with the very approximately cylindrical wall of a borehole. The borehole is normally filled with a fluid (such as a water-based mud) that if conductive provides an electrical conduction path from the formation surrounding the borehole to the buttons.
Many variants on the basic imaging tool design shown are known. In some more or fewer of the pads 11 may be present. The numbers and patterns of the buttons 12 may vary and the support arms also may be of differing designs in order to achieve particular performance effects. Sometimes the designers of the tools aim to create e.g. two parallel rows of buttons located on the pad one above the other in use. The buttons in the lower row are offset slightly to one side relative to their counterparts in the row above. When the signals generated by the buttons are processed the outputs of the two rows of buttons are in effect lain over one another. As a result the circumferential portion of the borehole over which the buttons 12 of a pad 11 extend is logged as though there exists a single, continuous, elongate electrode extending over the length in question.
Another type of resistivity logging tool is employed during so-called logging while drilling (LWD) operations. This logging tool may include only a single button.
In general in operation of a tool such as resistivity tool 10 electrical current generated by electronic components contained within the cylinder 14 spreads into the rock and passes through it before returning to the pads 11. The returning current induces electrical signals in the buttons 12.
Changes in the current after passing through the rock may be used to generate measures of the resistivity or conductivity of the rock. The resistivity data may be processed according to known techniques in order to create (typically colored) image logs that reflect the composition of the solid and fluid parts of the rock. These image logs convey much data to geologists and others having the task of visually inspecting and computationally analyzing them in order to obtain information about the subterranean formations.
In use of a tool such as that shown in FIG. 1 the tool is initially conveyed to a chosen depth in the borehole before logging operations commence. The deployed location may be many thousands or tens of thousands of feet typically but not necessarily below, and in any event separated by the rock of the formation from, a surface location at which the borehole terminates.
Various means for deploying the tools are well known in the mining and oil and gas industries. One characteristic of many of them is that they can cause a logging tool that has been deployed as aforesaid to be drawn from the deployed location deep in the borehole back towards the surface location. During such movement of the tool it logs the formation, usually continuously. As a result the image logs may extend continuously for great distances.
Generally the logging tool is conveyed to the depth at which logging commences supported on wireline, i.e. strengthened cable that is capable of both supporting the mass of the logging toolstring (that may be several tens or even hundreds of kilograms) and conveying electrical signals between the logging tool and a surface location. Thus the functions of wireline, that are in themselves well known in the logging art, include suspending the logging tool while it is being deployed for use; withdrawing the tool towards a surface location during use; transmitting electrical or electronic commands to the toolstring to effect chosen operations; and telemetering data, from which the logs are constructed, from the tool to the surface location while logging occurs.
Other designs of logging tool are autonomous in the sense of not being permanently connected to a surface location by wireline. Such tools include on-board power sources and memory devices. As a result they can be conveyed to downhole locations for example shielded within drillpipe and then exposed at the end of a length of drillpipe so as to log a formation surrounding the borehole while the drillpipe is withdrawn to a surface location. Such types of logging tool do not telemeter data in real time and instead store it in the on-board memory devices for downloading after retrieval of the logging tool to a surface location.